Three strategies are in common use for reducing the error rate of a digital transmission.
The first consists in an error correcting code without a return path (sometimes called forward error control or FEC). A defect of this error correcting strategy is that it is poorly adaptable and this may be objectionable for certain types of channel (tropospheric channels, or channels being jammed). For example in a tropospheric channel, various channel parameters such as mean signal to noise ratio, coherence band, or coherence time, remain fixed on a small scale only. If these parameters are observed over several hours, several days, or several months, it can be seen that they are subject to non-negligible fluctuations. Thus, a code designed to operate ideally under given parameters will probably be ineffective at certain times of day or at certain times of the year. This strategy has the advantage of ensuring a transmission delay which is fixed. However it is ineffective against bursts of errors unless it is associated with interlacing which suffers from the drawback of increasing transmission delay.
A second strategy is the automatic repeat request (ARQ) strategy which requires the use of a return path, with this strategy being based on detecting errors and on requesting repeats when necessary. It may be defined as follows:
data is sent in packets, and not continuously;
each packet contains data symbols accompanied by check symbols; and
the receiver makes use of the check symbols to detect whether an error has occurred in transmission. It then either accepts the block of data in the packet or it requests retransmission. Thus the ARQ strategy uses an error detecting code whereas the FEC strategy uses an error correcting code. An error detecting code is generally simpler to decode than is an error correcting code.
This strategy gives rise to additional redundancy constituted by retransmission. Thus although the redundancy specific to the coding scheme is fixed (and specified in terms of a code ratio), retransmission redundancy is a random variable. Total mean redundancy is specified in terms of an effective code ratio. The main drawback of this strategy arises when transmission conditions are bad (a very noisy channel) so that the error detecting code is quickly swamped and the number of repeats is very high, thereby increasing transmission delay. To mitigate this problem, it is present practice to associate ARQ strategy with an error correcting code, thereby providing the third strategy.
The third strategy is referred to as a "hybrid" strategy since it combines error correcting code and the ARQ procedure. The code used no longer serves merely to detect errors but also to correct them within the limit of the code's error-correcting capacity. The strategy is then as follows:
an encoded sequence is transmitted;
the receiver has a decoder which attempts to decode the sequence as received. A definite criterion is used for measuring the confidence that may be had in the decision of the decoder;
if the criterion is satisfied, then the decoded sequence is assumed to correspond to the sequence as transmitted, otherwise the receiver requests a repeat.
This hybrid ARQ strategy is described in various publications which make use either of block codes or of convolution codes decoded by the Viterbi algorithm. However, with such codes, it is not possible to obtain an arbitrarily small error rate while keeping the code ratio fixed. It is then possible to use a convolution code which is decoded by a sequential decoding algorithm. The best is doubtless the stack algorithm which, in addition, is capable of providing an indication concerning the reliability of decoding.
One of the drawbacks of strategies that automatically request repeats (the second and third above-mentioned strategies) is the ever increasing delay in data transmission due to successive repeats, thus requiring the transmitter to possess buffer memories of very large size, with data being lost if the memories overflow. This increasing delay means that these methods are unsuitable, in particular, for use in transmitting telephone calls.